This invention relates to the field of AM stereophonic reception and, more particularly, to means of controlling the cosine corrector circuit of a C-QUAM.RTM. stereo radio receiver during reception of unsatisfactory signals.
In the field of AM stereophonic receivers, the requirement for compatibility means that the envelope of the received signal must be the same as the envelope of a monophonic system having the same intelligence signal inputs. In the C-QUAM system, the envelope is therefore 1+L+R, but the phase modulation is the same as the phase modulation of a pure quadrature (non-compatible) signal. In a typical receiver, the intelligence signals may be detected as L cos .phi. and R cos .phi. where .phi. is the angle whose tangent is [(L-R)/(1+L+R)]. In order to restore the original L (left) and R (right) signals, a "cosine correction signal" is used. This correction signal may be derived from the received signal in any of several ways, but if the receiver signal is very weak, very noisy, or contains an appreciable element of co-channel interference, the derived correction signal may be inaccurate and could even introduce more distortion than it removes or could increase the noise components in the signal.
In a U.S. Pat. No. 4,159,396, assigned to the assignee of the present invention, a relatively simple solution to this problem was disclosed. Briefly, a switching circuit was added to a receiver, with the capability of disenabling the cosine correction function. The switching circuit was controlled by the output signal of a comparator which compared the level of the AGC circuit output with a reference voltage (DC). When the AGC signal went below the reference voltage, an electronic switch was activated to open the path of the cosine correction signal. The result of this procedure was to remove the correction signal during weak signals, when its presence would do more harm than good. However, the AGC signal turned out to be a poorer source of signal information for this purpose than had been expected. Also, the abrupt transition may be perceptible and may introduce even more distortion, particularly if the AGC signal happens to hover around the reference level for even a brief interval.